The present invention relates to industrial control systems used for real-time control of industrial processes, and in particular to a programmable logic controller with input/output (I/O) modules that can communicate directly with the World Wide Web.
Industrial control systems are special purpose computer systems used in controlling industrial processes. Under the direction of a stored control program, a programmable logic controller, being part of the industrial control system, reads inputs from a series of input modules and writes outputs to a series of output modules. The input modules read signals from sensors associated with the industrial process and the output modules provide signals to actuators and the like in the industrial process. The inputs and outputs may be binary, that is on or off or analog, providing a value with a continuous range or more complex I/O devices like motor controllers and the like.
Industrial control systems differ from conventional computer systems in that they provide highly reliable operation and deterministic real-time control. In part, this requires both that data communicated between the programmable logic controller and the input and output modules be transmitted in a predictable sequence. This may be provided by use of an I/O image table which is populated with complete sets of input or output data before control decisions are made and before outputs are forwarded to the I/O modules.
Often it is desired to monitor an industrial process remotely. The development of the Internet and the Web has raised the possibility of using Web servers and browsers to implement a remote human/machine interface that could be used on any browser enabled computer. Current proposals for providing Web access to an industrial control system envision a Web server module that may plug into a common backplane to communicate with the programmable logic controller. Input and output data is communicated between the I/O module and the programmable logic controller and selectively communicated, under the control of the programmable logic controller, between the programmable logic controller and the Web server module.
Such a Web server module may be inappropriately expensive for simple control applications, particularly with regard to the necessary reprogramming of the control program executed by the programmable logic controller to coordinate communication with the Web server module. This coordination process can significantly add to the processing burden of the programmable logic controller.